1. Field of the Invention
Embodiments of the present invention relate generally to disk drives and, more particularly, to systems and methods for encoding track and servo sector addresses in such drives.
2. Description of the Related Art
A disk drive is a data storage device that stores digital data in concentric tracks on the surface of a data storage disk. The data storage disk is a rotatable hard disk with a layer of magnetic material thereon, and data is read from or written to a desired track on the data storage disk using a transducer head, i.e., a read/write head, that is held proximate to the track while the disk spins about its center at a constant angular velocity.
To properly align the read/write head with a desired track during a read or write operation, disk drives generally use a closed-loop servo system that relies on servo data stored in servo sectors written on the disk surface when the disk drive is manufactured. These servo sectors form “servo wedges” or “servo spokes” from the outer to inner diameter of the disk, and are either written on the disk surface by an external device, such as a servo track writer, or by the drive itself using a self servo-writing procedure. The read/write head can be positioned with respect to the data storage disk by using feedback control based on servo information read from the servo wedges with the read element of the read/write head. The servo sectors provide position information about the radial and circumferential location of the read/write head with respect to the disk surface in the form of servo patterns.
A typical servo pattern consists of a preamble field used to synchronize the timing of the read channel and to adjust the signal amplitude, an area that provides track number and servo sector number used for coarse positioning of the read/write head, and servo bursts for fine positioning of the read/write head relative to a specific data storage track. The pattern typically includes a field for enumerating each servo sector and a field for enumerating each data storage track, where the servo sector number provides the circumferential position of the read/write head and the track number provides the radial position of the read/write head.
Due to the large number of servo sectors commonly found on a data storage disk, i.e., several hundreds, and the very large number of tracks found on a data storage disk, i.e., hundreds of thousands, fields enumerating the servo sector and track numbers occupy a significant portion of storage space on the disk that could otherwise be dedicated for user data. Because the information in these fields is largely redundant, with most bits remaining unchanged from track to track and from sector to sector, schemes have been developed in the art for minimizing the size of these fields
In some schemes, the field for servo sector number is eliminated entirely and is replaced by an index bit that serves as an index mark to designate a particular servo sector for each track. The index bit is set to an index value, e.g., 1, for one servo sector per track, the index bit being set to a null value, e.g., 0, for all other servo sectors on the track. A significant drawback to this approach is that, when the position the of the read/write head is unknown, the time required to reestablish a known position of the read/write head is, on average, one half revolution of the disk. Because head location is frequently unknown during normal operation of a disk drive, for example whenever switching between surfaces of a drive, this is a significant drawback.
Other schemes combine the track address and servo sector address into a single smaller field. In some cases such schemes introduce ambiguities in the actual position of the read/write head, since a portion of the unique address information is sacrificed in order to reduce the size of the track address/servo sector address field. Positional ambiguity is highly undesirable in the context of reliability of disk drive performance. In other cases, such schemes have the added constraint that the position of the read/write head can only be determined when it is positioned on the same track for some number of servo sectors, which also increases the time necessary to determine head position.
In light of the above, there is a need in the art for a system and method of encoding track addresses and servo sector addresses so that a minimal portion of a disk drive storage medium is used while allowing head position to be quickly determined.